The influence of the presence of H-2 on the physico-chemical properties of a Pd(10 wt.%)/gamma-Al2O3 catalyst was investigated. Catalysts were treated under methane combustion conditions with various amounts of H-2 in the feed. Samples obtained were characterized by BET, CO-chemisorption, XRD, XPS and/or ToF-SIMS techniques. The effect of H-2 is essentially to sinter the palladium phase and to help remove surface chlorine coming from the palladium precursor. In presence of low amounts of H-2, O migration phenomena are suggested to occur to maintain the surface of the palladium particles in an oxidized state while, as the H-2 concentration increases, metallic palladium progressively forms in the bulk. Above a threshold value, the H-2 concentration becomes too high, metallic palladium is formed on the surface. (C) 2006 Elsevier B.V. All rights reserved.